My DX5e tx has one red and then three green LEDs that indicate the condition of the batteries. Right-most green = new, middle green = not so new, etc. Seems like after ~1.5 hours of use new alkaline batteries drop down to the second green light. Not wanting to risk failure, I've been replacing the batteries as soon as they are on that second green light. Am I being overly cautious? If I go ahead and use them till they get to the 3rd green light can I be confident the thing will keep working? What happens if the red light comes on - is that a failure light, or a "pending failure, get right down" light? The manual isn't much help on this...

I use my alkaline batteries in my DX5 until the last green light is on when I turn on the transmitter, then I change them.
Said a different way, if I turn on the TX and the 2nd green light goes on, I fly, to the point where the 3rd green LED lights during the flight. Next time I use it, if the 2nd green LED comes on at the start of the flight, I'll still use it.

I had it go red during a flight (was flying a micro, was not worried about pushing it), and when it went red I think the TX shut down for a moment.

ALL rechargeable NiMh including Eneloops DO NOT have a top starting voltage of 1.5v like alkaline. They top out ~1.4v and quickly drop to 1.2v (nominal voltage). But they hang in there a LONG time. Expect the top light to go out quickly but it should be nice and stable after that for a good while. The gauge is calibrated for alkaline.